In response to stress, cellular damage must first be recognized then a signal transduced to the nucleus to install the correct gene expression program. In addition, the cell must coordinate its stress response with other organelles in the cell such as the ER and mitochondria. In particular, the mitochondria are a critical regulatory node for the stress response. In response to excessive cellular damage, the mitochondria undergo extensive fragmentation followed by mitochondrial outer membrane permeability (MOMP), which triggers the release of sequestered pro-apoptotic proteins from the mitochondria. MOMP is considered the point of no return for execution of the programmed cell death (PCD) pathway. My laboratory has identified cyclin C as a factor that connects changes in gene expression to mitochondrial fission and PCD. Cyclin C, along with its cyclin dependent kinase partner Cdk8, associates with the RNA polymerase holoenzyme to regulate transcription. In particular, cyclin C-Cdk8p are repressors of many stress responsive genes. To relieve this repression, cyclin C, but not Cdk8p, translocates from the nucleus to the cytoplasm in cells exposed to pro-oxidants and other stressors. In the cytoplasm, cyclin C interacts with the fission machinery and is both necessary and sufficient to induce extensive mitochondrial fragmentation. In addition, we identified a cytoplasmic role for cyclin C promoting PCD. These results suggest that cyclin C function connects mitochondrial fission to the cell death pathway. These activities are well conserved as we have demonstrated that the mammalian cyclin C also translocates to the mitochondria to direct fission and PCD in mammalian model systems. Therefore, the information obtained in this proposal will be applicable to higher systems as well. To elucidate the role of cyclin C in triggering mitochondrial fission and PCD, the following Aims are proposed: AIM 1. Define the role cyclin C plays in stress-induced mitochondrial fission. AIM 2. Determine the role mitochondria-ER junctions play in stress-induced fission. AIM 3. Dissect the requirement of cyclin C in PCD execution.